1. Field of the Invention
The present invention relates to an ink jet recording apparatus, and more particularly to an ink jet recording apparatus provided with a fixing heater for accelerating the fixation of ink deposited as a recorded image on a recording medium.
2. Related Background Art
The ink jet recording apparatus are attracting increasing attention in the field of recording apparatus, because of various advantages such as a very low noise level when recording and a capability of a a high density recording.
Also the ink jet recording apparatus has an advantage that it can use plain paper as the recording medium, but the fixation of the ink on a recording paper may be difficult depending on the combination of the recording sheet to be employed and the ink. Since the ink jet recording apparatus employs liquid ink as the recording material, the ink may not penetrate rapidly into the recording medium and therefore remain in liquid state thereon if the recording medium shows insufficient absorption to the ink.
In such a situation there may result smear by the transport members such as rollers, and said smear may be transferred to the succeeding recording medium, thereby significantly deteriorating the quality of recording. Also, in case of using a recorded sheet on which the ink is not sufficiently fixed not only a problem of smearing by the operator's hands but also a problem of ink spreading due to erroneously rubbing the image would be caused. Thus, the image quality is deteriorated. Such a drawback of smearing and spreading may also occur due to the mutual friction of the recorded media when the recorded media are stacked on a discharge tray.
In order to prevent such drawbacks and to facilitate the fixation of ink to the recording medium, there has been proposed to heat the recording medium with a heater, thereby accelerating the evaporation of water contained in the ink and thereby rapidly dry the ink on the recording medium. For the purpose of shortening the transport path of the recording medium and preventing the smear on the transport members, such a fixing heater is provided in the vicinity of the recording position of the recording head, for example in a platen for supporting the recording medium so that its recording surface would be flat in front of the recording head. Also said heater may be positioned at the upstream side of said recording position in the transport path for pre-heating the recording medium, since the ink absorption is enhanced if the recording medium is dried in advance.
The deterioration of the recorded image quality may also be caused by dewing in the ink jet recording apparatus. Under certain circumferential conditions, the moisture condensed by dewing in the apparatus may adhere to the recording medium, causing ink blotting. Such problems resulting from dewing cannot be prevented by the use of a fixing heater.
In order to prevent such dewing, the present inventors have looked to a different technical area, the electrophotographic technology, and have considered the application of a dew-preventing device employed in electrophotographic copying machines to the ink jet recording apparatus. For example, they tried a method of controlling the heating operation of a plurality of heaters provided respectively with a plurality of temperature sensors, single humidity sensor, paper feeding section, optical unit and fixing unit as described in Japanese Patent Laid-Open No.55-35390, and a method of removing moisture by blower as described in Japanese Patent Laid-Open No.56-80061 in addition to the above described heat control method.
However, these methods involve certain obstacles when simply applied to the ink jet recording apparatus, such as (1) the apparatus inevitably becomes bulky because of the use of plural antidewing heaters in addition to the fixing heater, and (2) the viscosity of the ink is influenced by the heat from the antidewing heaters so that the stability of the recording property is lowered. The experiments of the present inventors have revealed the cause and influence of dewing in the ink jet recording apparatus in the following manner.
In the ink jet recording apparatus, rapid and satisfactory fixation without warping in the recording medium is generally achieved, according to the experiments of the present inventors, in a temperature range of the recording medium of 80.degree. to 60.degree. C.
On the other hand, the recording head is maintained at 20.degree. to 40.degree. C. in consideration of the parameters influencing the stability of ink emission such as ink viscosity. This temperature is lower than the circumferential temperature.
Under such conditions, the face of the ink emission orifices of the recording head (hereinafter called emission surface) may cause dewing, because a high humidity atmosphere is created around the recording head, particularly in the vicinity of the emission surface, due to water evaporation from the recording medium and from the ink by heating for fixing.
In ink jet recording, it is generally important that the ink droplet is deposited at a desired position, in order to improve the image quality, and, for this purpose, it is required that the ink droplets for use in recording have constant flying direction and speed of emission with a uniform size.
The dewing on the emisison, surface, in particular, at the peripheral of the emission orifice generates uneven water drops thereon at the peripheral of the orifices, exerting random attraction to the ink droplets leaving the ink orifices and thus causing fluctuation in the direction and speed of emission of the ink droplets and in the size thereof. These facts lead to deterioration of the recorded image quality. Also the wet emission surface induces deposition of paper dusts and other dusts, which leads to the deterioration of recorded image quality.
It has therefore been recognized that the dewing in the ink jet recording apparatus could only be resolved by careful consideration of the fixing temperature, temperature of recording head considering the result and extensive time of experiments.